A quantitative method for the simultaneous measurement of propranolol and its major metabolite, 4-hydroxypropranolol, in human plasma has been developed. Following their separation by high pressure liquid chromatography as little as 1 ng of propranolol and 5 ng of 4-OH propranolol can be measured using a fluorimetric detector. Interference with the assay by other commonly used cardiovascular drugs was studied. The plasma concentrations of propranolol and 4-hydroxypropranolol were measured in hypertensive patients following oral administration of various dosages of propranolol.